This invention relates to a pattern detection method, and more particularly to a target pattern detection method which is especially useful when forming various types of fine patterns such as semiconductor integrated circuit patterns or magnetic bubble element patterns by using lithographic techniques.
For accurately forming a fine pattern at a desired position on the surface of a substrate by lithographic techniques, it is necessary to correctly place a mask having said fine pattern at a specified position above said substrate and irradiate the substrate through said mask with ultraviolet rays, electron rays, X-rays or ion beams (these irradiating rays or beams are generally referred to simply as light (rays) in this specification).
A semiconductor integrated circuit is formed by repeating exposure through each of the masks having different patterns. Therefore, if the mask is placed with even a slight deviation from the specified position, the respective patterns of wiring, electrodes and other elements formed on the substrate shift from the prescribed positions, making it difficult to precisely form a high-density semiconductor integrated circuit or magnetic bubble element.
The mask is set at its proper position by precisely aligning the pattern formed in the mask in with the target pattern previously formed on the substrate. The target pattern in not always directly formed on the substrate; it is often formed on an insulating film, conductive film, polycrystal silicon film or other types of film. In this specification, however, in order to simplify the explanation, said alignment pattern is described as being formed on the substrate.
Generally, monochromatic light is applied to the substrate and said target pattern is detected from the reflected light and aligned with the alignment pattern in the mask. However, monochromatic light applied to the substrate for detecting the target pattern gives rise to multiple interference in the resist film due to the reflection of light at the surface of the resist film formed on the substrate surface, and the reflected light from the target pattern on the substrate is also distorted by the effect of said multiple interference.
The thickness of said resist film can not be made uniform throughout the film but always varies to some extent, and said multiple interference in the resist film varies according to the film thickness, so that both the intensity and the phase of said reflected light vary according to the position on the resist film. This makes it difficult to detect reflected light (detection signal) from the target pattern precisely, and thus this has been one of the most serious impediments to the formation of high-density semiconductor integrated circuits or magnetic bubble memory elements.